1. Field of the Invention
The present invention relates to a control apparatus and a control program for controlling drive of a vibration-type drive device at start-up.
2. Related Background Art
Recently, vibration-type motors have been used not only as actuators in camera lenses, but also as actuators to drive calendars in watches, to drive photosensitive drums in copiers, etc. Further, examples of the shape (constructions) of a vibration member in vibration-type motors include a ring shape, a rod shape, a disc shape, and many others.
A general method of driving a vibration-type motor is shown in flowchart in FIG. 5 (see Japanese Patent Application Laid-Open No. 2001-25271, for example). Furthermore, FIG. 6 shows a pattern diagram of a frequency, a phase difference (the phase difference between an input voltage Va to the vibration-type motor (piezoelectric element) and an output voltage Vs of the sensor phase provided to the piezoelectric element), and the rotation number (speed) of a rotor (hereinafter referred to as the “rotor rpm”) when the vibration-type motor starts normally.
In FIG. 6, a horizontal axis shows time (ms), and a vertical axis shows frequency (kHz), rotor rpm, and the above-mentioned phase difference θa-s.
When the vibration-type motor starts, the frequency starts at the maximum frequency fmax within the frequency control range of the circuit (step S501 in FIG. 5), and with a predetermined sweep down rate R1 (Hz/sec) the frequency is swept down (step S502 in FIG. 5). Then, when an encoder or the like detects (by pulse detection) that the vibration-type motor (the rotor) has started to rotate (step S503 in FIG. 5), the phase difference θa-s is calculated (step S504 in FIG. 5). Here, when the calculated phase difference θa-s exceeds a predetermined phase difference P2 (step S505 in FIG. 5), the frequency sweep is stopped (step S506 in FIG. 5).
Subsequently, with a frequency f1 where the frequency sweep stopped serving as a reference, the motor is driven to a target point while controlling the frequency so that the phase difference θa-s falls between the above-mentioned phase difference P2 and a predetermined phase difference P1.
However, there is a fear that, when trying to start the vibration-type motor in a highly humid environment, it will not start on the first try, or it will only run at a very slow speed. This problem arises because, when the vibration-type motor is left in high humidity, trace amounts of moisture attach to minute gaps between the frictional surface between the vibration member and the rotor. This causes the vibration member and rotor friction coefficient to drop, reducing the torque generated when the motor starts. In particular, when using a SUS material, aluminum or other friction materials which have superior wear resistance and involves extremely little wear, the decrease in torque occurs more easily.
When the motor is in a low-torque state such as described above, in the conventional control method, sometimes the vibration-type motor cannot be started normally. That is, as shown in FIG. 7, even when the frequency is swept down from a high frequency (fmax) upon motor start-up and the friction amplitude of the vibration member has increased to a sufficiently large value V1 (frequency is f1), the rotor rpm (N1) is almost 0 due to the moisture and the like on the frictional surface.
In this case, the drive pattern is such that as the frequency is further dropped from the frequency f1, it eventually sweeps down to the minimum frequency (fmin) where the drive ends.
Here, since the vibration-type motor is driven by the frictional force between the vibration member and the rotor, the frictional heat on the frictional surface becomes extremely high, and when driven in a steady state the temperature on the friction surface reaches hundreds of degrees. However, when moisture intervenes on the frictional surface, within a short time period after the vibration-type drive device begins to start, the frictional heat generated on the slide surface is also very small and the conventional control method cannot eliminate the moisture.
As a common solution to the situation where moisture is present on the slide surface, the pressure applied to the contact surface between the vibration member and the rotor is increased so as to raise the surface pressure on the contact surface to thereby eliminate the moisture, or the frictional surface is roughed (creating concaves and convexes) to minimize the influence of the moisture.
However, in the vibration-type motor, simply raising just the applied pressure increases the load on the frictional surface, and thus increases wear. Moreover, since a load also bears on the bearing that receives the reaction force when the pressure is applied, the durability of the bearing deteriorates and damage to the bearing increases. Furthermore, when the slide surface is simply roughened, this increases the wear of the frictional surface, which detracts from the durability of the vibration-type motor.